This invention relates to semiconductor devices, and more particularly to contact and metallization systems for silicon semiconductor devices. Still more particularly, this invention relates to a single metal contact and metallization system for silicon semiconductor devices.
The requirements for a material or system of materials to provide ohmic contact to and metallization stripes from semiconductor devices is extremely stringent from a mechanical, electrical, and chemical view. Obviously, first criteria must be that the material provide good electrical conductivity and ohmic contact to the semiconductor region. In standard planar types of semiconductor devices and integrated circuits, the material must not only make good ohmic contact to the silicon but good mechanical contact to the silicon and to the silicon dioxide regions surrounding the contact area. Not only must the material bond well to silicon dioxide, but also particularly in multilevel systems to silica glasses or glasses of other types should have good adherence to the material upon deposition thereon. Preferably, the material has a temperature coefficient of expansion closely matching the material in which it must be in contact. For manufacturing reasons, the material should be easy to deposit by standard evaporation or sputtering techniques and be easily patterned by etching or similar techniques. Since, in certain instances, it will be necessary to bond either gold or aluminum wires to the contact or metallization regions, these materials must be readily bondable to the material by standard thermocompression or ultrasonic bonding techniques. In use, the material should be mechanically strong, corrosion resistant, and not subject to hillocking, electromigration, or similar thermal or electrical stress defects.
The only single metal previously found at all suitable for contacts in interconnections in silicon planar transistors and integrated circuits is aluminum, which material has been widely utilized for these purposes. The three most common problems attendant to the use of aluminum are: (1) pitting of the silicon in the silicon contact area, (2) electromigration, and (3) its thermal expansion characteristic relative to that of silicon and/or silicon dioxide. The latter characteristic results in hillocking of the metallization stripes, which can crack the dielectric material, particularly in multilevel structures. To some degree, the problems of electromigration and etch pitting of the silicon have been alleviated by the addition of small amounts of alloying impurities such as copper or silicon, into the aluminum. In many cases, it is desirable that gold wires be bonded to the aluminum metallization, and in such case, there is a further problem in that intermetallic compounds of gold and aluminum are formed. One of these intermetallic compounds is purple in color and, hence, has been referred to as "purple plague", in that intermetallic compounds may result in extremely poor mechanical and electrical characteristics. Another problem attendant to the use of aluminum is its apparently high reactivity with silicon dioxide, which can result in further electrical degradation of the device characteristics.
To overcome some of the defects resultant from the use of aluminum, multi-metal systems have been suggested, which systems usually include gold. Gold is well known as a good electrical conductor, and is extremely resistive to ordinary corrosion. However, it does not adhere well to silicon or silicon dioxide and, more importantly, it readily diffuses into the silicon and results in a lowering of the carrier lifetime. In fact, gold is ordinarily diffused into devices as a charge carrier lifetime "killer", where required. To overcome these problems, it has been necessary to use gold only in combination with barrier metal layers, thereby resulting in requirements of a multilayer metallization system, which system requires additional processing steps contrasted with a single metal system.